Semiconductor Interface Emits UV Light in Presence of Water

Photonics SpectraFeb 2016
PHILADELPHIA — Just a few molecules of water can cause a complex oxide material to emit intense UV light, a discovery that could have bearing on chemical sensors and optical computing.

The phenomenon was observed by a team led by researchers at the University of Pennsylvania, who were studying a lanthanum aluminate film on a strontium titanate crystal. Reproducing the experimental conditions, they realized that water molecules on the surface played a role in emission of UV light from inside the material.

In the presence of a water molecule on its surface, lanthanum aluminate film on a strontium titanate crystal emits UV light from its interior. Illustration not to scale. Courtesy of Drexel University.
The team turned to theory to help interpret the results.

"Dissociation of water fragments on the oxide surface releases electrons that move to the buried interface, cancelling out the ionic charges," said Fenggong Wang, a researcher at Penn. "This puts all the light emission at the same energy, giving the observed sharp photoluminescence."

This is the first report of the introduction of molecules to the surface controlling the emission of light — of any color — from a buried solid-surface interface, according to the researchers.

"In landmark discoveries, this interface between two electrical insulators has been shown to have an electrically conducting state, one that can be altered by water on the surface of lanthanum aluminate, and also exhibits superconducting and ferromagnetic ordering," said Jonathan Spanier, a professor at Drexel University who also took part in the study. "But this discovery is quite remarkable because we uncovered a chemical reaction at the surface that prompts the emission of light from the interface within — and we are able to turn it off and on again. Amazingly, we can also make it stronger by increasing the distance between the molecules and surface and the buried interface, by using thicker films, for example."

"We suspect that the material could be used for simple devices like transistors and sensors. By strategically placing molecules on the surface, the UV light could be used to relay information--much the way computer memory uses a magnetic field to write and rewrite itself, but with the significant advantage of doing it without an electric current," said Mohammad Islam, a former member of Spanier's team and now a professor at the State University of New York at Oswego. "The strength of the UV field also varies with the proximity of the water molecule; this suggests that the material could also be useful for detecting the presence of chemical agents."